We know that good stamps are always rare because of high demand and limited supply. Even postal department works a lot on absorbing excess supply. We can make our collection even rare and precious if we add some faulty or erroneous stamps in it. It is once again catch me if you can kind of concept because well established collectors generally have some private arrangement to get these kind of stamps but still if you keep on visiting GPO more often you can certainly have an access to faulty stamps. To begin with lets first of all try to understand different types of errors which a stamp can carry:
Perforation Error: This is the most common kind of error which a stamp may carry.

Source: http://setenantsofindia.blogspot.com/search/label/Error
Design Error: This kind of error may happen because of wrong spelling, wrong subject etc.
Value Error: This error happens when the stamps are priced incorrectly.
Double Impression: We can also call this error as double printing
Missing Colour: Certain times you can observe the same stamp with different shades. This is known as missing colour stamp.
There may be some more errors on the stamps but these are some of the commonly cited errors. So keep collecting and add value to the collection.